Aliens in a spaceship approach the Earth and decide to conquer it and begin to attack the Saotome Research Institute. Their allies at the Science Fortress Lab are still repairing the Great Mazinger, so no immeadiate could come. Musashi Tomoe is killed when fighting against Grangen. With the Getter Machine Bear destroyed, Getter Robo could no longer be used. Fortunately, a new Getter Robo was ready to be used, Getter Robo G. Benkei Kurama is recruited as a new pilot for the robot.

Great Mazinger and co. fight against Bong. However, in spite of destroying Bong; the machine alien was just bait for Pikadron to attack the Great. Even when Getter Robo G helps fight against it, Pikadron only grows stronger after absorbing their energy attacks. Great's pilot Tetsuya finds out that Pikadron can be destroyed when it is soldified, and using a new weapon destroys Pikadron. Both the Mazinger and Getter teams get together to grieve the loss of Musashi.